The present invention relates to steering controls and more particularly relates to a hydraulic control valve for actuating steering brakes and clutches of a track-laying vehicle.
Track-laying vehicles normally comprise two tracks, one on either side of the vehicle, which have drive sprockets engaged therewith. Power is transmitted to the drive sprockets by respective drive assemblies which each include a clutch and a brake. When the vehicle is travelling straight ahead, both clutches are engaged and both brakes are disengaged so that both drive sprockets are driven at equal speeds by the vehicle engine. If it is desired to turn the vehicle to one side or the other, one or the other of the clutches is disengaged so that the power from the engine is applied solely to one sprocket, thus resulting in the vehicle turning at a gradual radius. If it is desired that the vehicle turn sharper, the non-driven sprocket is braked with the amount of braking force applied, determining the sharpness of the turn.
Heretofore, it has been the practice to control these brakes and clutches hydraulically by means of right- and left-hand control valves which each include a valve plunger which acts through a spring, when depressed, to shift a unitary clutch and brake control spool against clutch and brake control pressure to sequentially effect clutch disengagement and brake engagement. This type of control has the disadvantage that a relatively stiff spring is required between the plunger and the control spool in order to maintain modulation while overcoming the reaction forces exerted on the control spool. U.S. Pat. No. 2,941,639, issued to Christenson et al. on June 21, 1960, discloses a clutch and brake control which exemplifies the prior art controls.